LED light sources differ significantly from incandescent light sources in that they typically produce light, the colour of which does not change significantly with their brightness. This is particularly apparent when an LED light source is used to replace a dimmable incandescent lamp. Depending on the brightness of the light, incandescent lamps produce light with a colour temperature between 1800K, when the lamp is deep dimmed, and 2700K when the lamp is at full brightness or even up to 3000K for an undimmed halogen lamp. During dimming the colour temperature of an incandescent follows the so-called black-body curve. In contrast to incandescent lamps, LEDs have an almost constant colour temperature of for example 3000K or 3500K independent of the dimming level.
To overcome this perceived problem, it is known to mimic an incandescent light by using a mixture of LED light sources which emit different spectral contents of light. For each brightness level, set by for example a wall-dimmer or remote control, the mixture is adjusted to mimic an incandescent light source. The solution is generally referred to as “Tunable White” or “Correlated Colour Tracking”.
It is possible to use different combinations of LED light sources to achieve the same effect. Some examples of combinations of coloured light sources, known as primaries, which have been used are: warm white combined with amber; cold white combined with amber; red combined with green and blue; and warm white combined with red and green. A warm white LED may typically be one having a colour temperature of 3000±100 K; conversely, a cold white may typically have a colour temperature of 3500±100 K. Although their Colour Rendering Index (CRI) may be of different quality, in principle any combination can be used, as long as the colour coordinates of the primary light sources in the XY colour plane cover the relevant part of the black body curve.
Warm or cold white combined with amber are particularly convenient combinations since, firstly, only two primary types of LEDs are required and so only two drive currents need to be adjusted. And secondly, both primaries are already located on the black-body curve, and as a result inaccuracies in the mixing do not result into colour deviations that appear unnatural.
As will be described in more detail hereinbelow, more than one LED may be used in series, for one or both of the primaries. Such a series arrangement is generally referred to as a string. In order to keep the cost of the driver low, the two LED strings will typically be supplied by a single output switching LED driver.
In known arrangements, the two strings are arranged in parallel. An example is shown in FIG. 1. This arrangement 100 which has a first string 110—in this case a single LED which may be for instance an amber LED, and a second string 120, which may be for instance white LEDs. The current being supplied from an LED driver 150, which may be either a linear type or switching type, is directed into both strings, and the fraction which is directed towards the first string is controlled by a controller 130 which uses some sort of regulated analogue current source circuit 140. Due to a difference in total forward voltage between the strings, the power efficiency may be expected to be low whenever both strings are simultaneously conducting current. For this reason, at maximum light output at which the white LEDs conduct full current, the current through the amber LED will be reduced to, or almost to, zero. Thus the amber LEDs do not contribute to the maximum luminance output from the light source, and at maximum brightness, all the light to come from the white LEDs. It will be apparent, that this is not an ideal solution, from a cost point of view, since the cost-efficiency of such a system, in lumens-per-dollar, is lower than would be the case, were the amber LED also to be contributing.
In another known arrangement, an example 200 of which is shown in FIG. 2, the current is switched to either of the first string 210 or the second string 220 in a sequential manner, by means of switches 240 and 245 under the control of controller 230. The problem of reduced power efficiency may be avoided, because the switching LED driver may be arranged to sequentially adapt to the individual forward voltages. However, this results in a complicated switching LED driver which may also be specific to a particular arrangement of LED strings and thus incompatible with standard switching LED drivers. Further, since LED operating efficiencies (measured in lumens/watt) are generally highest at constant current, buffer capacitors may be required in parallel with the LEDs to achieve the highest efficiencies.